OBJECTIVE: Evidence links depression and stress to more rapid progression of HIV-1 disease. We conducted a randomized controlled trial to test whether an intervention aimed at improving stress management and emotion regulation, mindfulness-based stress reduction (MBSR), would improve immunological (i.e. CD4+ T-cell counts) and psychological outcomes in persons with HIV-1 infection. METHODS: We randomly assigned participants with HIV-1 infection and CD4 T-cell counts >350 cells/μl who were not on antiretroviral therapy in a 1:1 ratio to either an MBSR group (n = 89) or an HIV disease self-management skills group (n = 88). The study was conducted at the University of California at San Francisco. We assessed immunologic (CD4, c-reactive protein, IL-6, and d-dimer) and psychological measures (Beck Depression Inventory for depression, modified Differential Emotions Scale for positive and negative affect, Perceived stress-scale, and mindfulness) at 3, 6 and 12 months after initiation of the intervention; we used multiple imputation to address missing values. RESULTS: We observed statistically significant improvements from baseline to 3-months within the MBSR group in depression, positive and negative affect, perceived stress, and mindfulness; between group differences in change were significantly greater in the MBSR group only for positive affect (per item difference on DES-positive 0.25, 95% CI 0.049, 0.44, p = .015). By 12 months the between group difference in positive affect was not statistically significant, although both groups had trends toward improvements compared to baseline in several psychological outcomes that were maintained at 12-months; these improvements were only statistically significant for depression and negative affect in the MBSR group and perceived stress for the control group. The groups did not differ significantly on rates of antiretroviral therapy initiation (MBSR = 39%, control = 29%, p = .22). After 12 months, the mean decrease in CD4+ T-cell count was 49.6 cells/μl in participants in the MBSR arm, compared to 54.2 cells/μl in the control group, a difference of 4.6 cells favoring the MBSR group (95% CI, -44.6, 53.7, p = .85). The between group differences in other immunologic-related outcomes (c-reactive protein, IL-6, HIV-1 viral load, and d-dimer) were not statistically significant at any time point. CONCLUSIONS: MBSR improved positive affect more than an active control arm in the 3 months following the start of the intervention. However, this difference was not maintained over the 12-month follow-up and there were no significant differences in immunologic outcomes between intervention groups. These results emphasize the need for further carefully designed research if we are to translate evidence linking psychological states to immunological outcomes into evidence-based clinical practices.